Tuesday, January 20, 2015

DOD FX 25 envelope filter lowpass filter modification

This is a fairly straightforward modification to change the DOD FX25 factory bandpass filter function into either just a lowpass filter or a switchable bandpass/lowpass function.  It really brings a lot of functionality by letting the low end come through the pedal, you can now use the same pedal for guitar OR bass equally.  This is just a mod for the filter section, the other modifications that most people do is on the envelope detector, which I am going to leave factory at this point, IMO it does what it does well enough to leave alone, YMMV though.  This pedal is somewhat infamous from some high profile users: Flea from RHCP and Bill Laswell, the NYC master producer and dub bass legend.

The filter section is based largely on the National Instruments LM13600 datasheet, Figure 14, voltage controlled state variable filter:
In the DOD implementation of the circuit they only brought out the bandpass filter to the outside world, the lowpass filter output has always been in there but was unused in the factory circuit design.  The modification simply changes the default connection between the pedal output circuit and the filter to one or both of the available filter outputs. The pedal I have is what I'll call 'Rev G', two knob with green sparkle paint and gray silkscreen on the top.  There are several different versions of this pedal that have slight differences, they all work in a similar manner so this mod is applicable to any of them.  It seems like the rev G version had higher production numbers and is more common than the rest?  Here is the schematic I am using with the mod on it:


You can also add a switch to select between the two filter outputs, using a SPDT switch.  Wire the center lug to the output capacitor and the two end lugs to the bandpass and lowpass filter output points.  The pedal shell is relatively cramped so I opted to use the lowpass output only and not try to stuff an additional switch into the pedal.  If you wanted to have an indicator for the filter you could use a bi-color LED and a DPDT switch to change LED colors when you switch filter outputs.  Here is the board pic _after_ modification:


One wire was added, so it really is a simple mod to do.  Also note the rev G silkscreen and the opamp socket added to U1.  Sockets are a great thing to add once you have the pedal opened, it allows you to swap out opamps to experiment with different tonal colors.  In this pedal there is only one opamp so it was also a simple mod to do, only half of the opamp handles the audio signal, the other half is for the envelope.  In the pic I'm using a rare, vintage early 80s Japanese made 6458D dual opamp, a relative of the commonly used 4558, a staple in the guitar pedal world.  These are a bit thicker, richer and darker than the stock opamp, in this pedal a later production 1458 from the 90s.  Here are some closeups of the connections:

 For the output connection I de-soldered the near leg of the capacitor C7 and bent it upwards so it was parallel with the board and floated off of it a little bit.  Butt solder a wire to the exposed lead to make the connection.  If you want you can add a dab of superglue to keep the wire held down, I used a solid core wire so it stayed in place fairly well.

For the input connection I was able to sneak the end of the wire under the resistor lead between the resistor body and the hole where it goes into the PCB.  It's a single sided board so there is no via to connect to, doing the connection this way is a bit tricky but you don't have to run a wire all the way to the bottom side of the PCB.

1 comment:

Unknown said...

Are you familiar with the FX25b pedal? Is this doable to this pedal aswell?? Thank you!